This invention relates in general to optical transceivers and, more particularly, to laser rangefinders which are capable of adapting to a wide dynamic range of target reflection intensities.
A laser rangefinder is an optical transceiver which is capable of measuring the distance between the rangefinder and a target in the distance. The laser rangefinder includes a laser light source or laser transmitter, and further includes a photodetector or laser receiver. Typically, the laser transmitter irradiates the target with incident laser light pulses and the laser receiver receives the reflected light pulses. A signal processor in the rangefinder records the time between transmission of a pulse by the laser transmitter and reception of the reflected pulse back at the rangefinder receiver. The signal processor then uses this time difference to calculate the distance between the rangefinder and the target.
It is known to provide laser rangefinders with the ability to adjust the intensity of the laser beam transmitted from the laser transmitter to the laser target. This capability is useful during military training exercises to avoid eye injuries to friendly personnel by attenuating the laser beam transmitted to the target. It is also desirable to attenuate the intensity of the laser beam generated by the laser rangefinder to the minimum level required in order to avoid detection by enemy personnel.
When the target is distant from the laser rangefinder transmitter, the reflected beam from the target is relatively weak. However, when the target is close to the rangefinder, the reflected beam from the target is very strong. In the latter case, the reflected beam can be so strong as to cause saturation and/or malfunction of the photodetector in the receiver of the laser rangefinder. This, of course, is not desirable.
One common technique for attenuating the transmitted laser beam is to insert an optically absorptive material or filter in the laser beam's path. Unfortunately, this technique results in heat build-up in the absorptive material so that the material acts as a lens and distorts the collimation of the laser beam. Moreover, this approach provides only fixed attenuation of the transmitted laser beam's intensity.
Another approach is to provide a partially reflective element in the transmitted laser beam's path. In this instance, the partially reflective element is designed to pass only a small portion of the transmitted signal and to reflect the remainder to a path where it is absorbed by an optically absorptive material. This approach solves the aforementioned lensing problem, but still only provides a fixed attenuation.
U.S. Pat. No. 4,530,600 issued to Lopez for a "Variable Attenuator For Optical Transceiver" teaches continuously varying the attenuation of the laser beam transmitted by an optical transceiver. To accomplish this, a polarization rotation device such as a Pockels cell and a second polarizer are inserted in the path of the transmitted laser beam. Changing the amount of polarization rotation of the polarization rotation device which is positioned before the second polarizer varies the intensity of the laser beam transmitted from the transmitter. In this case, the polarization rotation device and the second polarization device together act as a variable attenuator.
U.S. Pat. No. 4,227,146 issued to Morgan for a "Visible And Infrared Intensity Limiter" teaches the use of a Pockels cell to form a limiter which cuts off transmission of a laser beam when the intensity of the beam reaches a predetermined amplitude.
Unfortunately, prior attenuation techniques do not adequately protect the laser rangefinder receiver from overload and saturation when the receiver receives a very strong reflected light signal from a very close target. In that situation, receiver overload damage may occur and even if damage does not occur, the receiver may take an undesirably long amount of time to recover from the overloading reflected signal. Rangefinder performance can thus be significantly degraded.